This kind of self-tacky packaging films are largely used in families, restaurants and hotels for packaging foodstuffs, and exhibit effects of preventing foodstuffs from an evaporation of water during a storage in refrigerator, an evaporation of water during a heating in a microwave oven, a diffusion of flavor or taste or getting scented with other odors during a storage, or getting dusty during a use for business. Particularly, the popularization of a microwave oven in the recent years has much promoted a demand of the use for these kinds of packaging films. That is, the films are used for tightly packaging food containers usually made of ceramics or glass for the purpose of preventing an evaporation of water and a loss of taste in a microwave oven.
The packaging films used for such purposes are required to be tightly adhesive to the containers and simultaneously to exhibit a good tackiness property between the films each other. Further, the packaging films must be small in thermal shrinkage and exhibit neither thermal fusion nor whitening in a microwave oven (hereinafter, these properties are referred to as "a microwave oven suitability").
As conventional self-tacky packaging films, those using polyvinylidene chloride, polyethylene or polyvinyl chloride as a main starting material are known.
However, the films made from polyvinylidene chloride are disadvantageous in that they show a great shrinkage when heated, and this type of films are more expensive than the other two types of films. On the other hand, the films prepared from polyethylene have a fault that perforations are readily formed when the films are brought to a high temperature in contact with an oily or fatty material such as edible meats, fried foods, etc. The films prepared from polyvinyl chloride have a problem that they show a whitening phenomenon when brought into contact with boiling hot water.
Accordingly, none of these conventional self-tacky films can be said to be satisfactory in the point of a microwave oven suitability.
Many of such self-tacky packaging films are furnished to users in the form of a film having a width of 20 to 45 cm and a thickness of 10 to 20 .mu.m rolled on a core material made of paper or the like and placed in a case made of paper or the like. The film is withdrawn from the case to a desired length and cut out by means of a blade called "saw blade (edge)" fixed on the case, after which the cut-out film is used for sealing the opening of a food container or the cut section of a food, by utilizing the self-tacky property of the film itself.
As the saw edge used for cutting the film, a simple edge prepared by punching out an iron plate having a thickness of about 0.2 mm into a shape of saw is generally used. The case supporting the saw edge is usually a coated cardboard box having a basis weight of about 350-700 g/m.sup.2, which is very low in stiffness.
It is demanded to cut the film with such a simple cutting mechanism without difficulty. Actually, however, the cutting sometimes takes place at a position apart from the case or the saw edge.
Films made of polyvinylidene chloride, polyvinyl chloride or polyethylene as starting material have hitherto been used. In the case of polyvinylidene chloride film, if a tear appears at some positions of the film at the time of cutting, the tear extends so as to cut the film not along the saw edge but slantingly. In the cases of polyvinyl chloride film and polyethylene film, the film shows a great elongation at the time of cutting which deteriorated sharpness of cutting.
Although the films mainly made of polyvinylidene chloride or polyvinyl chloride have an appropriate flexibility (in this specification, flexibility is expressed in terms of Young's modulus) and therefore they are successfully used as domestic wrapping films, these films contain a large quantity of chloride, due to which they produce chlorine gas upon incineration. Further, the large quantity of plasticizer present therein is harmful. For these reasons, these films cannot be said to be satisfactory in environment suitability.
Thus, as a self-tacky packaging film made mainly of polypropylene, Japanese Patent Publication KOKOKU No. (JP-B-) 58-46216 has proposed a film obtained by compounding polypropylene with a specified quantity of polyethylene, JP-B-63-63578 has proposed a film compounded with a specified quantity of a nucleating agent together with atactic polypropylene, rosin or the like, Japanese Patent Application KOKAI No. (JP-A-) 4-328144 has proposed a biaxially stretched film obtained by compounding a polypropylene type copolymer with a specified quantity of a surfactant, and JP-B-2-14935 has proposed a method for producing a self-tacky packaging film by stretching a resin composition containing a tackifier.
Further, JP-A-6-32952 has proposed a film composed mainly of 4-methylpentene-1.
However, there has never been obtained a self-tacky packaging film simultaneously satisfying tackiness, a microwave oven suitability and "saw edge"-cutting characteristics and having an appropriate flexibility.